util: Introduce virBitmapNewString

util: Introduce virBitmapNewString

Our bitmaps can be represented as data (raw bytes for which we have
virBitmapNewData() and virBitmapToData()), human representation (list
of numbers in a string for which we have virBitmapParse() and
virBitmapFormat()) and hexadecimal string (for which we have only
virBitmapToString()).  So let's add the missing complement for the
last one so that we can parse hexadecimal strings.

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>